ALEX Mowatt missed a penalty and was sent off for his reaction in the aftermath as Karl Robinson’s first game in charge of Oxford United ended in a heavy defeat at Fratton Park.

The new head coach saw his side concede to Kal Naismith’s bundled finish from a corner in only the fifth minute.

But United went on to play some excellent football in a half-hour spell either side of half-time, forcing goalkeeper Luke McGee into two superb saves.

The pressure appeared to have told on the hour when Rob Dickie was shoved over for a penalty.

But after Mowatt’s spot-kick hit the woodwork he reacted to provocation from Nathan Thompson by striking the defender in the face.

The deserved red card was exploited by Pompey top-scorer Brett Pitman, who converted two passes from Jamal Lowe to seal their first home win of 2018.

Defeat left United heading into the Easter period five points above the bottom four in Sky Bet League One.

International call-ups limited Robinson’s options, but he handed Malachi Napa a first senior start to replace the injured Jon Obika, while Joe Rothwell made a welcome return on the bench.

The start of a new era began with a familiar problem – conceding from a set piece.

Just five minutes were on the clock when Naismith bundled in the game’s first corner from close range after Matt Clarke headed Connor Ronan’s delivery across goal.

It was a terrible start for the visitors, who had to weather an early storm which saw Naismith glance a header wide.

Aside from winning a couple of early corners, it took United until midway through the half to forge a chance.

Ryan Ledson and Napa combined to tee up James Henry, whose effort from 18 yards was tipped on to the crossbar by McGee.

After clearing the resulting corner, the hosts broke and Pitman cleverly positioned himself so Dickie caught him a yard outside the penalty area.

The centre back was booked, but Pitman could not increase the punishment, smashing his free-kick off target.

It landed in among the travelling supporters, who saw their side go on to dominate the game up to the half-time.

With Brannagan coming to the fore, United moved the ball with purpose and pushed Pompey backwards.

On 32 minutes McGee made another outstanding save, getting fingertips to turn Napa’s curling effort from 20 yards just round the post.

From the resulting corner United appealed for an equaliser, but referee Charles Breakspear adjudged Pitman had hacked Dickie’s header away before it crossed the line.

The visitors kept coming, though, and only a last-ditch defensive block prevented Henry from making it 1-1 after good work from Wes Thomas.

United were on top, but were almost caught out by a break on the stroke of half-time – Brannagan having to make an excellent block to thwart Pitman ten yards out.

The visitors remained in the ascendancy after the break with a series of corners.

One of them bore fruit on the hour, as Breakspear spotted a push on Dickie and awarded a penalty.

Mowatt took on the spot-kick duties, but despite sending McGee the wrong way his effort struck the goalkeeper’s left upright and bounced clear.

It instantly got much worse for the Barnsley loanee, who reacted to Thompson’s goading by striking the former Swindon Town defender.

Once it was spotted by the officials, there could be only one outcome – a straight red card.

The faint hope United had of getting back into the game disappeared altogether eight minutes later, when Pitman slammed in Jamal Lowe’s low cross to make it 2-0.

The pair combined again for the third goal ten minutes later.

United were carved open through the middle as Lowe charged through and rounded Simon Eastwood. Substitute Ricardinho appeared to have tackled the forward, who managed to square for Pitman to tap into an empty net.

The remaining 11 minutes were about damage limitation for United, who needed Eastwood to keep out a powerful effort from Pitman, searching for his hat-trick.

Portsmouth (4-1-4-1): McGee, Walkes, Whatmough, Clarke, Donohue, Thompson (May 87), Lowe, Close, Ronan (Evans 51), Naismith, Pitman.

Unused subs: Bass, Burgess, Kennedy, Chaplin, Haunstrup.

Booked: Thompson, Naismith, McGee.

Oxford Utd (4-4-2): Eastwood, Kane, Dickie, Mousinho, Smith-Brown (Ricardinho 77), Ledson, Brannagan, Mowatt, Henry (Mehmeti 77), Napa (Rothwell 66), Thomas.

Unused subs: Shearer, Martin, Ruffels, Ashby.

Booked: Ledson, Dickie, Brannagan.

Sent off: Mowatt.

Referee: Charles Breakspear (Surrey).

Attendance: 17,892 (1,050 visitors).